With the advent of silicon junction diodes and thyristors, solid state diodes and thyristors have steadily grown in power handling capacity to very high current and high voltage. In spite of higher power handling capacity, circuit designers are, however, constantly faced with situations where diodes and thyristors must be used in series or in parallel to provide the desired high voltage or high current characteristics for the circuit. One of the most important parameters in seriesing of such devices is the recovery charge (Q.sub.rr) of the individual devices. The recovery charge of each diode or thyristor in series must be matched with the other diodes and thyristors so that each device supports substantially the same amount of voltage, within specified limits. Otherwise, an individual device of the series having too high a recovery charge will support too much of the series voltage and be burned out, causing failure of the entire series circuit.
Heretofore, the recovery charge was controlled by a gold diffusion step after the diode or thyristor structure was made, typically by diffusion techniques. The gold diffusion changed the lifetime of the anode-base region and in turn, the recovery charge of the device, while leaving the forward voltage drop of the device substantially uneffected. One of the main problems with this procedure has been the controllability and reproducibility, particularly in high current devices (e.g. greater than 500 amps) where a large active area is present.
This application is considered an improvement on the method described and claimed in U.S. Pat. No. 3,933,527 granted Jan. 20, 1976 to the applicants and assignee of this application. Companion patents and patent applications are U.S. Pat. Nos. 3,809,582, 3,840,887, 3,852,612, 3,872,493, 3,877,977, 3,881,963, 3,881,964, and 3,888,701 and U.S. Patent applications Ser. Nos. 540,208 (filed Jan. 10, 1975) now U.S. Pat. No. 3,990,091, Ser. No. 581,255 (filed May 27, 1975), Ser. No. 639,337 (filed Dec. 10, 1975) and Ser. No. 667,791 (filed Mar. 17, 1976), all of which are assigned to the same assignee as the present application.
In U.S. Pat. No. 3,933,527, applicants teach irradiation of power diodes with radiation sources generally and electron radiation preferably to optimize the reverse recovery time while minimizing the forward voltage drop of the diode. The radiation dosage level preferably corresponds to between 1 .times. 10.sup.12 and 5 .times. 10.sup.13 electrons/cm.sup.2 with 2 MeV electron radiation. More preferably, the radiation dosage level corresponds to between 5 .times. 10.sup.12 and 2 .times. 10.sup.13 electrons/cm.sup.2 with 2 MeV electron radiation.
Applicants have now found that the recovery charge of power diodes and thyristors can be predictably tailored, with precision, by utilization of a different and more specific narrower range of radiation dosage corresponding to between about 1 .times. 10.sup.12 and 8 .times. 10.sup.12 electrons/cm.sup.2 and preferably between about 1 .times. 10.sup.12 and 5 .times. 10.sup.12 electrons/cm.sup.2.